The invention relates to a method for the transmission of data from one station to another by way of a data bus to which several stations are connected in parallel such that signals emitted by all the stations are linked to the data bus via an OR function in which all the stations receive the signals present on the data bus and at least some stations independently of one another access the data bus for a data transmission when data to be transmitted are present and the signal present on the data bus corresponds to the logic value "0" for a specified time period. For a data transmission a signal corresponding to the logic value "1", which identifies the condition of the data bus as "occupied", is transmitted periodically during a specified initial part period. A signal which corresponds to the value of the data bit to be transmitted is then transmitted during a specified second part period and then a signal corresponding to the logic value "0" on the data bus is transmitted during a specified third part period.
Such a method is known from U.S. Pat. No. 4,418,386, and uses a transmission medium which can transmit at least two logic states, for example a twisted pair of wires or a coaxial cable or also, for example, a glass fiber connection. This enables a data transmission system to be accomplished particularly inexpensively. However, in the case where several stations access the bus at the same time, which station finally retains the access to the bus must be resolved. This problem can be solved more easily if several parallel transmission lines are used for the data bus, but this then means higher expenditure for the connection which is also undesirable.
In the arrangement as described in the above-mentioned U.S. Pat. No. 4,418,386, stations which are ready to transmit wait for an initial period of time after the end of the last operation on the data bus to see whether the bus remains in the quiescent state, and if this is the case, a character is transmitted via the bus after a second period of time. This is achieved by the fact that after expiration of the second time interval a transmitting station brings the logic value "1" to the bus after it previously had the logic value "0" in the quiescent state. A certain time after this, a data character is transmitted and then the data bus returns to the quiescent state, in which case the transmitting station, or stations, monitors the state of the data bus again for a given time. All times or time intervals are dependent in this case on the internal clock generators of the respective stations which can exhibit relatively large scatters in their frequency. In order to obtain unambiguous states particularly in the case of simultaneous access by several stations, the individual times must, on account of these tolerances, obey certain conditions as pointed out in U.S. Pat. No. 4,418,386. This leads to troublesome and time-consuming control in the individual stations which therefore is also expensive.
U.S. Pat. No. 4,429,384 also describes a method and an arrangement in which the data is also transmitted via an individual transmission circuit, such as, for example, a twisted pair of wires or a glass fiber section. The transmission takes place in blocks, in which case a block is preceded by a start bit during which the data bus is brought to the logic value "1". The logic values of the data bits are represented in this case by different durations of a given signal state on the data bus, in contrast to the first-named arrangement as described in U.S. Pat. No. 4,418,386 in which the logic value of a bit is determined by the signal value itself on the data bus. In the case of the arrangement as described in U.S. Pat. No. 4,429,384 it is also necessary that the frequencies of the clock generators in the individual stations are defined within certain limits, in which situation several classes of frequencies are also possible for the clock generators. Then, however, at the beginning of a data transmission the transmitting station has to ascertain first of all to which class of clock frequencies the called station belongs. This transmission method, too, is cumbersome and relatively expensive.
It is the aim of the invention to outline a method of the type mentioned in the preamble which at low expense makes it possible to transmit data between stations with considerable clock frequency differences, without the clock frequency of the called station being known in the transmitting station at the start of a transmission. The invention achieves this aim by transmitting signals during three partial periods whose durations have a fixed relationship one to the other. In all stations the length of the entire period and hence the position of the partial periods within it is determined from the signal transfers on the data bus at the beginning of two successive first part periods and the state of the data bus is determined from the signal on the data bus during the first partial period and the logic value of the transmitted data bit is determined during the second part period.
Because of the fixed relationships between the part periods and because of the joining together without a pause and especially because in all stations the lengths or positions of the part periods are automatically determined practically independently of clock generators inside the stations, there are no synchronization problems. The part periods can be determined from the signal on the data bus by simple means, as is shown later. This creates a very inexpensive data transmission method which can also be profitably used in simple applications.
It is advantageous that the information on the data bus directly indicates whether the data bus is occupied because this occupancy information is uniquely allocated to a certain point within the period. This means that waiting times and special control sequences are not necessary or are very simple. In order that the length of the entire period for a data transmission can be simply measured at the beginning of a data transmission it is advisable, according to one embodiment of the invention, that at least one period without transmission of a bit should be included before the beginning of a data transmission. Because in this case the transmitting station generates a definite signal for occupancy of the data bus each time at the start of the first part period, the length of the transmission period can be determined from the first two successive signals of this kind, and is thus fixed at the beginning of the second period and can be used to evaluate data information in the second part period. This is based on the fact that at the end of a transmission the data bus constantly carries a signal corresponding to the logic value " 0" until a station begins a renewed transmission.
Another possibility consists in also maintaining the synchronization during the transmission pauses. In one embodiment of the invention this is achieved by subdividing the first part period into two sub-periods. During the first sub-period a signal corresponding to the logic value "1" is always transmitted on the data bus even outside of a data transmission while during the second sub-period a signal corresponding to the logic value "1" is transmitted via the data bus only during a data transmission. A station that is ready to transmit accesses the data bus only when the signal on the data bus in the second sub-period corresponds to the logic value "0". In this case, therefore, the first part period is subdivided into two regions the first of which is used for synchronization and the second for occupation of the bus. The generation of the signal corresponding to the logic value "1" on the data bus may permanently come from one of the stations which, for example, is available only for the transmission of this signal within the first sub-period, or the stations alternate in that after completion of a data transmission in the first sub-period each station continues to transmit a signal corresponding to the logic value "1" until another station occupies the bus and thus takes over the synchronization. In this case this new accessing station can continue to operate with the same period length in accordance with the preceding constant synchronization, but it can also generate a new period length. In the latter situation, of course, it is advisable once again to insert at least one period without transmission of a bit before the start of a data transmission.
A particularly suitable choice of the lengths of the part periods or the sub-periods is for the second and the third part periods or the sub-periods to have the same durations of 1/4 of the period in each case. Such a ratio is particularly easy to implement. To obtain the most trouble-free evaluation of the information transmitted via the data bus it is expedient that the status of the data bus and the logic value of the transmitted bit should be determined in each case approximately in the middle of the part period or sub-period concerned. Such determination or sampling times are also very easy to implement.
In many cases a transmitting station calls another station to collect information from the latter and transfer it to the transmitting station. For this, the desired other station can be addressed by the transmitting station through the data transmission and the transfer wish communicated. After this the initially transmitting station terminates the data transmission, and as soon as the data bus is signalled as being unoccupied, the previously addressed station begins the data transmission. In a further embodiment of the invention, however, there is also a simpler possibility as follows: even if, in the first part period of the second sub-period, a signal corresponding to the logic value "1" is transmitted by a first station via the data bus, a second station addressed by data sent from this first station transmits via the data bus, during the second part period of at least one period, a signal corresponding to the logic value of a data bit to be transmitted, and the first station during the second part period of this period feeds a signal corresponding to the logic value "0" to the data bus. It this way it is easy to conduct a dialogue between two stations.
Embodiments of a station for implementation of the invention method are also described.